In general, a satellite communication system uses complementary terrestrial components (CTC) such as repeaters, complementary ground components (CGC), or ancillary terrestrial components (ATC), to perform communication between a satellite and terminals. Examples of the satellite communication system include a satellite digital multimedia broadcasting (DMB) system, a digital video broadcasting—satellite services to handhelds (DVB-SH) system, and a geostationary orbit (GEO)-based mobile satellite communication system.
The satellite DMB system that already provides services additionally uses a terrestrial system that uses the same channel gap filler with a satellite and provides high-level audio and multimedia signals to a user that utilizes services. In this case, the same channel gap filler is used to effectively resolve a problem related to coverage in a shadow area. In order to provide these services, a frequency bandwidth used by a satellite and a frequency bandwidth used by a terrestrial system are optimized to a bandwidth in a range of 2630 to 2655 MHz.
The satellite DMB system includes a feeder link earth station, a satellite for broadcasting, a terrestrial repeater, and a terminal that receives services. A signal that is transmitted from the terminal is transmitted to the satellite through the feeder link earth station. At this time, for an uplink, a band (e.g., 14 GHz) for a fixed satellite service (FSS) is used. The satellite converts the received signal into a 2.6 GHz band signal, and the converted signal is amplified to have a predetermined magnitude by an amplifier in the repeater of the satellite and is broadcast to a terminal that is located in a service area.
It is required for the terminal to receive a signal transmitted from the satellite through a small antenna having low directivity. For this purpose, the terminal needs to have sufficient effective isotropic radiated power. Thus, the satellite needs to have a large transmitting antenna and a high-power repeater.
When the satellite transmits a 2.6 GHz band signal, a shadow problem occurs due to obstacles on a path from the satellite. In order to overcome this problem, at the time of designing a system, it is required to additionally provide a repeater that retransmits a satellite signal. The repeater allows the signal transmitted from the satellite to be transmitted to places where the signal cannot reach due to band obstacles, such as buildings. The repeater is divided into a direct amplification repeater and a frequency conversion repeater.
The direct amplification repeater only amplifies a 2.6 GHz band signal that is received from the satellite. The direct amplification repeater uses a low gain amplifier to prevent an unnecessary divergence from occurring due to signal interference between a receiving antenna and a transmitting antenna. The direct amplifier covers a small area at a distance of 500 m from the repeater on the basis of a line of sight (LoS).
Meanwhile, the frequency conversion repeater covers a large area at a distance of 3 km from the repeater, and converts a 2.6 GHz band signal transmitted from the satellite into another frequency band (e.g., 11 GHz) signal and transmits the converted signal to the terminal. In an environment where the two types of repeaters are needed, multipath fading occurs when two or more signals are received by the terminal.
As another example of the mobile satellite communication system, the DVB-SH system provides services to a terminal using a satellite for nationwide coverage, and services to the terminal using the CGC for an indoor environment and terrestrial coverage. The DVB-SH system provides a mobile TV service at a 15 MHz bandwidth of an S band on the basis of a DVB-H. In this case, the DVB-SH system uses a band near to a band used for terrestrial international mobile telecommunication (IMT) of an S band. Accordingly, integration with the terrestrial IMT and network reuse with a terrestrial system are easy, which results in decreasing installation costs.
The DVB-SH system considers a hybrid broadcasting structure with the terrestrial system. In order to resolve a signal interference problem between the satellite and the CGC and efficiently use a frequency, the DVB-SH system considers a structure in which a reuse factor is set to 1 with respect to a CGC cell in one satellite spot beam and to 3 with respect to the satellite spot beam. In this case, if using the satellite spot beam, 9 TV channels can be broadcast in nationwide coverage, and 27 channels can be broadcast through the terrestrial repeater in a downtown area or an indoor environment.
Finally, the GEO-based mobile satellite communication system has been developed in Mobile Satellite Ventures (MSV) and Terrestar in order to provide to a terminal a ubiquitous wireless broadband communication service such as an Internet access service and a voice conversation service in L and S bands. The GEO-based mobile satellite communication system uses a hybrid wireless network structure where the satellite and the ATC are coupled to each other and provides voice or high-speed packet services through the ATC, that is, a terrestrial system in a downtown area or congested area, and services through the satellite in the country or areas outside the downtown that are not covered by the ATC. Since the ATC uses a wireless interface such as the satellite, the GEO-based mobile satellite communication system has been developed such that satellite services can be provided without increasing complexity of the terminal.
All personal portable mobile satellite communication systems that will be developed use a satellite in the country or areas outside the downtown where a line of sight is secured are scheduled to provide services using a complementary terrestrial component, and using the complementary terrestrial component in the downtown area or an indoor environment where satellite signals are not secured. However, systems and signal transmission methods have been developed for the purpose of either a broadcasting service or a communication service, such as voice or data.
Thus, in order to provide a communication and broadcasting integrated service that is anticipated to be provided in the future to a user, it is required that the personal portable mobile satellite communication system effectively provides a communication service and a broadcasting service at the same time. Further, it is required to maximize frequency utilization by increasing spectrum utilization efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.